1. Field of the Invention
The present invention relates to a process for economically producing particles of crosslinked melamine resin having a uniform particle diameter. The particles will find use as an organic filler, slip agent for plastics film, and spacer for liquid crystal display units.
2. Description of Prior Art
It is known that particles of thermoplastic resin having a uniform particle diameter can be produced by the emulsion polymerization of vinyl monomer, as disclosed in Japanese Patent Laid-open No. Sho 59-59741 (59741/1984). These particles, however, are inherently poor in heat resistance and solvent resistance and hence are used only in extremely limited application areas.
For the production of particles of thermosetting resin, there have been proposed several processes. For example, Japanese Patent Publication No. Sho 62-18564 (18564/1987) discloses a process for producing fine spherical particles (0.005-100 .mu.m in diameter) of cured resin by the spray-drying or precipitation with stirring of a solution of an amino acid-modified product or precondensate of methylol melamine. Japanese Patent Publication No. Sho 62-20212 (20212/1987) discloses a process for producing clear, fine spherical particles of amino resin by mixing a precondensate of amino compound and formalin with a non-solvent in the presence of a dye. Japanese Patent Laid-open No. Sho 62-68811 (68811/1987) discloses a process which consists of performing condensation and curing on an aqueous precondensate of melamine and/or benzoguanamine and formaldehyde in the presence of alkylbenzenesulfonic acid in an aqueous solution containing a surface active agent, thereby giving an emulsion, and separating and drying the cured resin. Japanese Patent Publication No. Sho 56-42614 (42614/1981) discloses a process which consists of reacting benzoguanamine or a mixture of benzoguanamine and melamine with formaldehyde to give a soluble, fusible resin, immersing the resin in stirred water containing a protective colloid at 40.degree. to 60.degree. C. for 1 hour, and curing the resin at 60.degree. to 200.degree. C.
As mentioned above, emulsion polymerization merely gives rise to resin particles having poor heat resistance and solvent resistance.
Particles of thermosetting resin are superior in heat resistance and solvent resistance. Nevertheless, their production poses many problems. For example, the process needs a comparatively expensive raw material such as amino acid and also needs a spray dryer and high-speed stirrer. The process is inefficient because it has to be performed in a dilute solution and hence it gives rise to only a small amount of particles for the amount of solution processed and gives rise to the desired uniform particles only at a high production cost. In another case, the process needs a solvent in place of water and gives rise to comparatively coarse particles having uneven particle diameters. Moreover, the process needs an expensive raw material such as benzoguanamine and a special curing agent. In this case, the process is inefficient and uneconomical due to the dilute solution that can be processed. The process also needs a special stirrer such as colloid mill, homomixer, and disperse mill. All this is disadvantageous to the simple, economical production of the desired particles.